By definition, vapor pressure is the amount of pressure within a vapor or gas when the substance is in an equilibrium state. In other words, when a liquid or solid is in a closed container and some molecules evaporate while others return to the liquid or solid state, the pressure that can be measured within that container relates to the vapor. Vapor pressure is expressed in terms of atmospheres (atm) and may be affected by temperature changes, container size and molecular bond strength.

When water transforms into steam and the steam is trapped in a container, the water vapor pressure will increase until it reaches a point of equilibrium. At that point, the rate of evaporation is equal to the rate of condensation. In other words, when steam is captured in a closed area, the rise in temperature resulting from an increase in steam leads to condensation inside the container. The water molecules are trapped within the container walls and cannot escape. As a result, the molecules begin to collide and bond and return to a liquid state.

As temperature increases, the rate of evaporation also increases. Molecules will continue to evaporate until the point of equilibrium is reached. The point of equilibrium also is known as the saturation vapor pressure, because the vapor is completely saturated. The process of condensation commences once the point of equilibrium has been reached.

The size of the container in which the vapor is held also has an effect on pressure changes. The more vapor there is within a contained area, the more pressure there is within that same area. For illustration, imagine a growing crowd of people jammed into a small room. As more people enter the room, the people will get closer together, because the room size does not change. In the case of vapor, as more molecules enter a container, the closer together the molecules become and the more vapor pressure increases.

In addition to temperature changes and space constrictions, the type of bonds in the molecular structure can dictate whether the vapor pressure will be relatively high or low. Basically, the easier it is for the bonds to form, the quicker the rate of condensation will be and, thus, the point of equilibrium will occur relatively quickly. If the equilibrium state is reached quickly, the vapor pressure will be relatively low. Alternately, if the bonds are weak then the molecule will bond slowly. It will take longer for the rate of evaporation to equal the rate of condensation and the molecule will have a high vapor pressure.

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